Surface mountable semiconductor devices requiring heat dissipation are often mounted on insulated metal substrates (IMS) in order to efficiently remove the heat during operation. Such an insulated metal substrate usually comprises a heat dissipating metal layer, typically a flat sheet metal, covered by an electrically insulating dielectric layer. On the dielectric layer an electrically conductive layer is applied in order to form conductor paths for contacting the mounted semiconductor device. Even a thin layer of dielectric material with a low thermal conductivity however leads to a high thermal resistance in the assembly and thus to an increased junction temperature. This is in particular the case for small devices with a high thermal power dissipation which results in a high power density. Mounting SMT (surface mount technology) devices with an insulated thermal pad, i.e. a pad formed of a highly heat conducting material, on an IMS offers the opportunity to directly bring the thermal pad into good thermal contact with the metal substrate without the usual dielectric layer. To this end, the dielectric layer is not present or removed in the area underneath the thermal pad in order to directly thermally connect the thermal pad to the metal substrate by means of a highly heat conducting interconnect layer.
US 2011/0180819 A1 discloses a surface mountable semiconductor device having two electrical connection pads and a thermal pad at a bottom surface of a substrate on which a LED is mounted. The electrical connection pads and the thermal pads have the same height and are mounted via an interconnect layer to an electrically and thermally conductive layer on a PCB (printed circuit board) substrate. The thermal pad is thermally connected via the conductive layer with a heat release member arranged on the PCB substrate.
When mounting such a surface mountable semiconductor device on an IMS with a dielectric layer locally removed underneath the thermal pad, the interconnect layers between the electrical connection pad and the electrically conductive layer and between the thermal pad and the metal substrate require different amounts of material, like e.g. solder, due to the uneven substrate surface caused by the locally removed dielectric layer. This may lead to connection defects when mounting the semiconductor device on the substrate. Furthermore, a usual cost effective application of the interconnect layer by uniform deposition of a solder paste is not possible in such a case.